The xylene isomers metaxylene, orthoxylene and, in particular, paraxylene, are important chemical intermediates. Orthoxylene is oxidized to make phthalic anhydride which is used to make phthalate based plasticizers among other things. Metaxylene is oxidized to make isophthalic acid which is used in unsaturated polyester resins.
However, paraxylene has by far the largest market of the three isomers. The largest use of paraxylene is in its oxidation to make terephthalic acid. Terephthalic acid in turn is used to make polymers such as polytrimethyleneterephthalate, polybutyleneterephthalate (PBT), and polyethyleneterephthalate (PET). PET is made via condensation polymerization of terephthalic acid with ethylene glycol.
PET is one of the largest volume polymers in the world. It is used to make PET plastics (e.g., two liter PET bottles). It is also used to make polyester fiber which, in turn, is used to make clothes and other fabrics. Polyester fiber is used both as a homofiber, as well as a blended fiber, such as a blend with cotton. Given the large market for PET plastics and fibers, there is a substantial demand for high purity paraxylene. The demand for paraxylene is several times larger than the demand for ortho and metaxylene. The demand for paraxylene is also larger than the amount of paraxylene in the xylenes recovered as a by-product, such as the xylenes recovered from reformate (from catalytic reformers) and from pygas (from high temperature cracking to make light olefins).
Because the demand for paraxylene is so much larger than the demand for the other xylene isomers and is larger even than the supply of paraxylene in xylenes recovered as a by-product, it has been found that isomerization of xylene isomers is desirable to increase the amount of paraxylene production. Paraxylene is typically produced by reforming or aromatizing a wide boiling range naphtha in a reformer, for example, a CCR (Continuous Catalytic Reformer), and then separating by distillation a C.sub.8 aromatics rich fraction from the reformer effluent. This C.sub.8 fraction comprises near equilibrium amounts of ethylbenzene and the three xylene isomers, namely, para-, meta- and ortho-xylene. The paraxylene in this C.sub.8 aromatics fraction is separated by either crystallization or adsorption. Rather than simply returning the paraxylene depleted C.sub.8 aromatics stream to the refinery for a relatively low value use such as gasoline blending, the C.sub.8 aromatics stream which is depleted in paraxylene is typically further processed by passing it over a xylene isomerization catalyst in a xylenes isomerization unit. The resulting C.sub.8 aromatics stream, which now has an approximately equilibrium concentration of xylenes, i.e., a higher concentration of paraxylene, is recycled to the paraxylene separation process.
The xylene isomerization unit typically serves at least two functions. First, it re-equilibrates the xylenes portion of the stream. Thus, in effect, it is creating paraxylene from the other xylene isomers. Second, it transalkylates or hydrodealkylates the ethylbenzene to facilitate its removal from the C.sub.8 aromatics fraction. Since ethylbenzene boils in the same range as the xylene isomers, it is not economic to recover/remove the ethylbenzene by distillation, hence it is included in the C.sub.8 aromatics fraction that is fed to the paraxylene separation process. Ethylbenzene is in general an inert from a para-xylene production standpoint, except for those para-xylene production Complexes which utilize a xylene isomerization process where the ethylbenzene is converted to xylenes. However, as pointed out earlier, such a process is limited in it's ethylbenzene conversion by C.sub.8 aromatics equilibrium. Therefore, for those cases where ethylbenzene is an inert, it is highly desirable to remove as much ethylbenzene as possible per pass so that it does not accumulate in the recycle loop. If that were to occur, a bleed stream out of the para-xylene production loop would be necessary which would reduce para-xylene production. Thus, a critical function of the isomerization plant is to react-out the ethylbenzene by either hydrodealkylation or transalkylation/disproportionation depending on the type of isomerization process.
Current xylene isomerization technology is based on two types of processes, high pressure processes and a low pressure process. Furthermore, within the high pressure processes, there are two types of such processes. U.S. Pat. No. 4,482,773 and U.S. Pat. No. 4,899,011 are two references dealing with one type of the high pressure process, usually carried out at 150 psig and higher and in the presence of hydrogen. U.S. Pat. No. 4,584,423 is a reference dealing with low pressure isomerization, usually carried out at less than 150 psig, for example, between about 25 and 100 psig and in the absence of hydrogen.
In the '773 and '011 high pressure processes, a C.sub.8 aromatics-rich hydrocarbon feed is contacted with a catalyst containing a ZSM-5 zeolite, xylene isomerization is carried out simultaneously with ethylbenzene hydrodealkylation to benzene and ethane. The hydrogen/hydrocarbon feed mole ratio is between 2/1 and 4/1. In both these patents, the objective is to achieve high levels of ethylbenzene conversion to isomerize the xylene to achieve a higher content of paraxylene, preferably an equilibrium content of paraxylene and to have low xylene losses. In the '773 process, ethylbenzene conversion levels are about 60% and xylene losses are about 2% yielding an ethylbenzene conversion/xylene loss ratio of about 30/1. Similar values are achieved with the '011 high pressure process, but at ethylbenzene conversions of about 70%. For both these high pressure processes, the catalyst system is very xylene selective.
In U.S. Pat. No. 4,482,773, the catalyst used comprises platinum and magnesium on a ZSM-5 zeolite. The preferred catalyst is a HZSM-5 (H meaning that the ZSM-5 is predominately in the hydrogen form) with a preferred crystal size of 1-6 microns. The examples in U.S. Pat. No. 4,482,773 disclose a H.sub.2 /HC feed mole ratio of 2/1 or higher.
The high pressure process of U.S. Pat. No. 4,899,011 is similar to the '773 process but uses a dual catalyst bed system. The objective is to hydrodealkylate ethylbenzene in the first catalyst layer and complete the isomerization of xylenes in the second layer. The catalyst for both layers is a Pt containing ZSM-5, without any Group IIA metal such as Mg. The Pt ranges from 0.05-10 wt. %. The crystal size of the first layer is 1 micron minimum compared to 0.1 micron maximum for the second layer. In addition, the top layer is a more acidic ZSM-5 than the second layer. Operating conditions for the '011 process are 400.degree.-1000.degree. F., 0-1000 psig, 0.5-100 WHSV, and a H.sub.2 /HC feed mole ratio of 0.5/1 to 10/1.
The catalysts for both U.S. Pat. No. 4,482,773 and U.S. Pat. No. 4,899,011 have good xylene isomerization activity as determined by the Paraxylene Approach To Equilibrium (PXAPE) which reaches values of 100-103%. A PXAPE of 100% indicates that the paraxylene concentration on a xylene basis is at equilibrium. The catalyst of both processes is based on ZSM-5. In the case of the '773 process, the catalyst contains Pt and possibly Mg. The catalyst has a silica/alumina ratio of about 50/1 to 100/1 and a crystal size of 1-6 microns. In the case of the '011 process, the catalyst bed consists of two catalyst layers, each of which contains Pt. Catalyst crystal size and acidity differ with the top catalyst having a crystal size of 2-4 microns and the bottom layer having a crystal size of 0.02-0.05 microns. In addition, as mentioned above, the top layer is more acidic than the bottom layer.
It should be noted that within the high pressure xylene isomerization process technology, there is a sub-type of process where the objective is to eliminate the ethylbenzene by converting the ethylbenzene to xylenes. However, high levels of ethylbenzene conversion as in the '773 and '011 patents are not achieved with this type of process, as the ethylbenzene concentration is limited by the equilibrium concentration on a C.sub.8 aromatics basis.
In addition to U.S. Pat. No. 4,899,011 and U.S. Pat. No. 4,482,773 discussed above, two other patents of interest are U.S. Pat. No. 4,467,129, issued Aug. 21, 1984 to lwayama et al., and U.S. Pat. No. 4,899,010, issued Feb. 6, 1990 to Amelse et al.
U.S. Pat. No. 4,467,129 is very similar to the '733 and '011 processes, in that ethylbenzene is converted by hydrodealkylation and uses a mixture of mordenite and a ZSM-5 which contains rhenium. A ZSM-5 containing Mg and Re is disclosed. Platinum is not a catalyst component. The process operates at 572.degree.-1112.degree. F., a pressure of 0-1370 psig, and a H.sub.2 /HC feed mole ratio of 1-50/1. The examples show a temperature of .about.700.degree. F., a pressure of 165 psig, and a H.sub.2 /HC feed mole ratio of 4/1. We estimate the WHSV at 3.5.
U.S. Pat. No. 4,899,010 is also an ethylbenzene hydrodealkylation/xylene isomerization process. It is based on the hydrogen form of a borosilicate equivalent of ZSM-5 known as AMS-1B. The catalyst contains 0.1-1.0 wt. % Pt. Operating conditions are 700.degree.-1000.degree. F., 0-100 psig, and a H.sub.2 /HC feed mole ratio of 0.25-5.0. Ethylbenzene conversions are about 25-28% and the ethylbenzene conversion/xylene loss ratio is about 29.
In the low pressure xylene isomerization process, which operates without any hydrogen present, ethylbenzene conversion is achieved by the disproportionation of ethylbenzene. The products of this disproportionation reaction are benzene and di-ethylbenzene, a C.sub.10 aromatic. Ethylbenzene is also converted by another reaction, namely, by transalkylation with the xylenes. This latter reaction produces benzene and di-methyl-ethylbenzene, also a C.sub.10 aromatic. This reaction with xylenes results in an undesirable loss of xylenes. Another reaction mechanism which contributes to xylene loss is the disproportionation of xylenes to produce toluene and trimethylbenzenes, a C.sub.9 aromatic. All these reactions are a function of the catalyst acidity. Operating conditions are such as to achieve ethylbenzene conversions of about 25-40%. However, the xylene losses are high, on the order of 2.5-4.0%, resulting in an ethylbenzene conversion/xylene loss ratio of 10/1. Thus, at 40% ethylbenzene conversion, the xylene losses are 4%. Furthermore, high levels of ethylbenzene conversion, in the range of 50-70% are not practical as the temperature required to achieve these levels of ethylbenzene conversion would be quite high. At 70% ethylbenzene conversion, the temperature required is about 60.degree.-70.degree. F. higher than that required to achieve 50% ethylbenzene conversion. In addition, the coking rate would also be substantially higher due to the higher operating temperature and higher level of ethylbenzene conversion. The net effect of operating at higher ethylbenzene conversion is a substantial reduction in the catalyst life.
One key goal of xylene isomerization catalyst development efforts has been to reduce xylene losses at constant ethylbenzene conversion, or to achieve higher ethylbenzene conversions while reducing the xylene losses.
U.S. Pat. No. 4,584,423, which describes a low pressure isomerization process, discloses that a Mg/ZSM-5 extrudate resulted in a 40% reduction in xylene loss when used in low pressure isomerization, and a Zn/ZSM-5 extrudate resulted in a 30% reduction in xylene loss when used in low pressure isomerization operating at 25% ethylbenzene conversion. However, the operating temperature for the reaction zone was higher relative to the base case using ZSM-5 catalyst. Operation to achieve 50% ethylbenzene conversion would have required even higher operating temperatures.
From a catalyst stability standpoint, the high pressure processes which operate in the presence of hydrogen have catalyst systems which are an order of magnitude more stable than the low pressure process which operates in the absence of hydrogen. For high pressure processes, catalyst stability is believed enhanced by using a catalyst which contains a hydrogenation/dehydrogenation metal component, such as platinum, and by using a high hydrogen partial pressure. The high hydrogen partial pressure is achieved by combining a high system/process pressure with a high hydrogen/hydrocarbon feed mole ratio, for example, 4/1. This is equivalent to a hydrogen concentration of approximately 80 mole %.
Accordingly, in a low pressure process, it would be desirable to achieve the performance parameters of the high pressure isomerization processes and achieve high levels of ethylbenzene conversion, while simultaneously achieving xylene isomerization and very low xylene losses, and achieving high stability for the catalyst.